Cavitation and morphological changes in polypropylene deformed at elevated temperatures

Polypropylene (PP) thick films were subjected to tensile drawing at various temperatures from the room temperature to 100 °C. Morphological alterations during drawing were followed by wide-angle X-ray scattering, small-angle X-ray scattering, and scanning electron microscopy (SEM) of sectioned and etched samples, volume strain measurement, and light transparency measurement at various level of strain. The morphological observations were paralleled with stress-strain determination. Samples drawn at 25 and 40 °C undergo severe cavitation contributing to their volume increase up to 90-95%. The volume increase contributes greatly to the engineering strain. PP drawn at 70 and 100 °C does not cavitate. At the strain up to 1.2, a high lamellae orientation is observed in SEM, whereas the 2D WAXS patterns show in contrary circular diffraction rings indicating low orientation of crystals. The rotation of lamellae toward drawing direction is associated with reverse rotation of chains in crystals due to fine chain slips. These two rotations in opposite directions counterbalance resulting in a much weaker crystal orientation than expected from the SEM images. Noncavitating samples retain their translucency up to a high strain.